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Opal-like deposits spotted on Mars indicate the planet may have been wet for a billion years longer than previously thought, report US researchers.

If confirmed, the findings, by NASA's Mars Reconnaissance Orbiter (MRO), could have significant impacts for whether the planet was suitable to host life.

Dr Scott Murchie of John Hopkins University and colleagues report their discovery of a new category of hydrated minerals on Mars in the current issue of the journal Geology.

"Water may have existed as recently as two billion years ago," says Murchie. "It extends the time range for liquid water on Mars, and the places where it might have supported life."

The silica-based deposits are the third - and most significantly, the youngest - type of water-containing mineral discovered on Mars.

The oldest hydrated materials are clay-like phyllosilicates, which formed more than 3.5 billion years ago when volcanic rocks bathed for long periods of time in water.

Later, hydrated sulfates formed when salty and occasionally acidic water evaporated.

Hydrated silicates, commonly known as opals, formed when liquid water altered materials created by volcanic activity or meteorite impacts on the planet's surface.

They have been found in Gusev Crater by the rover Spirit and by the MRO in the Valles Marineris canyons and other relatively young areas of Mars.

"We see numerous outcrops of opal-like minerals ... around the rim of Valles Marineris and sometimes within the canyon system itself," says research team member Ralph Milliken with NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

Water played a key role

Some of the hydrated silica deposits spotted by MRO were mixed with iron sulfates, which formed when long-standing acidic water evapourated. Milliken and his colleagues think the water played a role in the formation of the opals.

"What's important is that the longer liquid water existed on Mars, the longer the window during which Mars may have supported life," says Milliken.

Also this week, NASA said it is powering down science instruments and heaters on its Phoenix lander, which has been analysing ice and soil samples from the planet's northern polar region.

With the Martian summer in retreat, energy to run the solar-powered chemistry lab is running low.

"If we did nothing, it wouldn't be long before the power needed to operate the spacecraft would exceed the amount of power it generates on a daily basis," says project manager Dr Barry Goldstein of JPL.

"By turning off some heaters and instruments, we can extend the life of the lander by several weeks."

Phoenix landed five months ago to determine if Mars had the right ingredients to support life.

Glaciers on the surface of Pluto could explain the mysterious frozen world's youthful skin. Also: most Earth-like planet ever found orbiting a Sun-like star, and more support for the Standard Model of particle physics.